Heretofore, as upright assemblies for fork lift trucks, there have been known liner types in which an inner mast received in an outer mast is elevationally guided by a liner, and roller types in which an inner mast received in an outer mast is guided by rollers. Such upright assemblies of the inner mast receiving type have the feature of a forward field of view from the driver's seat greater than that of upright assemblies of the type where an outer mast and an inner mast are aligned in the lateral direction (right and left directions) of the fork lift truck.
FIG. 1 shows a conventional liner type upright assembly. In this upright assembly, liners 5 are interposed between the front and rear inner faces of an outer mast 1 and the front and rear outer faces of an inner mast 3 received in the outer mast 1, a side liner 7 is interposed between the side inner face of the outer mast 1 and the side outer face of the inner mast 3, and longitudinal and lateral moments acting on the inner mast 3 are supported by the liners 5, 7. A lift roller 11 attached to a lift bracket 9 is made to roll in a space inside the inner mast 3.
However, conventional liner type upright assemblies have problems in that sliding resistance between the masts 1 and 3 is large which adversely affects the lifting speed of the mast, thereby reducing efficiency. Further, the liners 5, 7 wear severely and it is hence necessary to frequently replace the liners, a complicated task that increases maintenance costs.
On the other hand, conventional roller type upright assemblies as disclosed in Japanese Patent Publication No. 49-49548 and Japanese Utility Model Laid Open No. 54-159575 are also known. FIGS. 2-4 show an upright assembly equivalent to those disclosed in these official gazette documents. As shown in the drawings, front and rear outer mast rollers 13 and 14 rolling on the front and rear outer faces of the inner mast 3 are attached to the upper end of the outer mast 1 through roller brackets 15, and small and large inner mast rollers 17 and 18 rolling on the front and rear inner faces of the outer mast 1 are attached to the lower end of the inner mast 3 through L-shaped roller brackets 19. Longitudinal moments are generated by the mast rollers 13, 14 and 17, 18. In this case, forward moments acting on the inner mast 3 are much larger than rearward moments, and the inner mast roller 18 at the rear side has a larger diameter. As shown in FIG. 2, lateral moments are supported by bringing the side face of the rear inner mast roller 18 having a large diameter into contact with a thick portion 1a formed at the inner corner of the outer mast 1. A lift roller 11 attached to a lift bracket 9 is so received as to roll in the inner space of the inner mast 3.
However, since the inner mast rollers 17 and 18 are disposed directly under the inner mast 3 in this roller type upright assembly, the lift roller 11 at the lowermost portion of the lift bracket 9 when it is disposed at its lowest position as shown in FIG. 3 must be at least the dimension D above the rear inner mast roller 18. Particularly, since the roller pin 21 of the inner mast roller 18 is only supported by one end on the L-shaped roller bracket 19, the roller pin 21 needs to be strengthened by increasing its diameter with the result that the diameter of the inner mast roller 18 is unavoidably increased. Thus, the lift roller 11 on the lift bracket 9 must be disposed at a considerably higher position than the lower end of the roller bracket 19 which is substantially the lower end of the inner mast 3, increasing the resultant overall height H of the upright assembly, with the result that there are cases where such an assembly cannot be used for loading/unloading in structures having low ceilings.